Membrane for sound reproducing devices



July 5, 1955 sz 2,712,360

MEMBRANE FOR SOUND REPRODUCING DEVICES Filed May 6, 1954 FIG. I.

IN ENTOR v E UGE IVE RE/SZ hired. States Patent Ofiice MEMBRANE 56RSGUND REPRGDUCING DEVICES Eugene New York, N. Application lday 6, 1954,Serial No. 428,076 11 (Jlaims. (Cl. 18l32) This invention relates to thereproduction of sound with a high degree of fidelity and it hasparticular reference to electrodynamic speakers with a conical or funnelshaped membrane. The main object of the invention is a materialimprovement of the quality of the sound produced by speakers having suchmembranes.

High fidelity reproduction of tones and sounds pro duced by musicalinstruments, especially by a plurality of instruments with or withoutaccompanying human voices entails a very close reproduction of complexoscillations covering a wide frequency range without selectivepreference for single notes or certain frequency ranges and one of themain difficulties in obtaining high fidelity resides in the fact thathigh frequencies have inherent properties which are sufiicientlydifferent from lower frequencies to cause all electromechanical systemsto show a marked preference for one group of frequencies while othergroups of frequencies are attenuated or suppressed. The high frequencieshave to a certain extent the character of beams, propagated in straightlines without bending or spreading, while low frequencies spread easilyaccording to an exponential law and may be guided along surfaces withoutmaterial loss.

It has therefore been proposed to provide separate sound producingelements and separate sound channels for the higher and lower frequencyranges. This has been partly successful, but entails great complicationsof the structure and introduces many additional problems of sounddistribution. If sound reproducers with two sound reproducing elementsfor two different ranges of frequencies are used without separatechannels, for instance two conical membranes radiating in the samedirection, either driven by the same voice coil or by separate voicecoils, the success is much less marked as the two concentric soundreproducing elements are of different efficiency either because one ofthe elements lacks means for preventing pressure equalization betweenits two sides or because the double suspension for the two elementsinterferes with the transmission of the excursions of the voice coil toat least one sound reproducing element.

It has also been proposed to treat the membrane in different ways indifferent zones so as to produce a different rigidity in the central andin the peripheral zone, using one zone mainly for the higher and theother for the lower frequencies. This entails the formation of a pliablezone between the two zones of different rigidity impairing thetransmission of the larger excursions (the base tones) to the outerzone.

According to the invention, the single funnel shaped or conical membraneof a loud speaker is completely divided into two separate parts,separated by a small gap along a plane which intersects its axis, saidparts or sections being joined and coupled in a novel manner. Thecoupling consists preferably of tube like symmetrical bodies, arrangedastride of the gap separating the two sections of the membrane andprojecting from the surface of the membrane substantially at rightangles thereto, which bodies are attached to the membrane sections.

Patented July 5, 1955 The tube like bodies may consist of an open meshtextile fabric, such as gauze, or of another fibrous material, stiffenedto such an extent that the free oscillations of the tube like bodies orof the elements of which they consist are not within the range ofaudible frequencies.

The gap must be of such narrow size that pressure equalization betweenthe front and the back of the membrane through the gap, especially forthe lower frequencies is prevented, while it must be sufficiently wideto permit independent movement of the high frequency reproducing sectionrelatively to the low frequency section.

The term cone or conical membrane used throughout this specification isintended to include not only frusto conical membrane shapes, but allflaring or funnel shaped bodies which are centro symmetrical and areformed by or contained within a body of rotation with a generatrixformed by a straight or curved line, one end of which is closer to theaxis of rotation than the other end.

From the above it is seen that one object of the invention consists inobtaining a major improvement of the quality of the reproduction ofmusical and other sounds by using different sections of the samemembrane for the reproduction of different frequencies, high fidelitybeing thus obtained without the complication due to the arrangement ofmultiple reproducing elements or devices.

A further object of the invention consists in an improved distributionof the different frequencies which are emitted by the same membrane.

A still further object of the invention consists in using asectionalized membrane which is sufficiently rigid for the transmissionof frequencies in the lower range from one section connected with thevoice coil of the device to the other section, but which permits theseparate use of a small stiffened section of the membrane for thereproduction of frequencies of the higher range.

A still further object of the invention consists in a divided membranewhose parts or sections are coupled with each other in a special manner,permitting the transmission of one group of frequencies from a firstsection of the membrane to a section sec on thereof, while the saidfirst section may oscillate independently and radiate another group offrequencies.

Further objects of the invention will in the following detailedspecification.

The invention is described with reference to the so companying partlydiagrammatic drawings illustrating one embodiment thereof. It is howeverto be understood that the embodiment of the invention which has be bestexplained and the best mode of applying said principle. The shape andthe constructional details illustrated are to be considered asillustrative and are not limitative, and a departure from theillustrated example of a speaker membrane is therefore not necessarily adeparture from the principle of the invention.

In the accompanying drawings:

Fig. l is a perspective view of a loud speaker membrane according to theinvention.

Fig. 2 is a diagram representing a section through the membrane along aplane of symmetry passing through the axis.

Fig. 3 is an elevational side view of the membrane looking towards thelarge opening of the membrane.

Fig. 4 is a perspective of a detail.

The speaker membrane 10 according to the invention may be approximatelyconical, as illustrated in the drawing, or may have the shape of ahyperboloid, or of any other body of rotation or of any other morecomplex body with a flaring mouth and a narrow end or apex 11, to whichnarrow end portion the cylindrical J carrier 14 for the voice coil 12 isattached in any well known manner. The membrane consists in most casesof a cellulosic material, like paper, or of other fibrous material,selected in accordance with the effects to be obtained, using approvedprinciples. The membrane is held by a sound board in a manner notspecifically shown.

This membrane ll), as shown in Figs. 1-3, is completely divided into twosections and 18 along a plane, indicated at b-b in Fig. 3 in dots anddashes, which plane in the example illustrated, is perp- *ular to themain axis aa of the conical membrane. two sections 15 and it; areseparated by a small gap 16, the section 15 extending from the apex tothe gap 16, while the section extends from the gap outwardly.

The gap 16 is as small as possible, it need only be wide enough to allowthe central section 15 to perform high frequency oscillationsindependently of and relatively to the outer section 18. Its width musthowever be so small that the passage of pressure equalizing sound waves,especially of the lower frequencies, from the front to the rear of themembrane through the gap is blocked. Pressure equalization betweenoscillations produced at the front and at the back of the membranerespectively, which show a phase difference of 180 and therefore canceleach other out and reduce the efficiency materially is thus prevented.

To transmit the oscillations imparted to the voice coil 12 which ismerely connected with the section 15 to both sections of the membrane,the section 15 of the membrane is coupled with the outer section 13 bymeans of a number of tubular coupling members 24), cylindrical tubesbeing shown by way of example. These coupling members may consist of anyfibrous material, for instance of textile material, preferably a wovenfabric such as fine gauze, or any other open mesh fabric. While thematerial in itself is not of primary importance, the stiffness of thecoupling members in various directions must be carefully selected andthis is best possible by selecting as a material a suitable open meshwoven fabric like gauze which is available with any number of threadsper square unit. Careful selection is necessary because any freeoscillation which may occur within such a coupling member should beentirely outside the limits of audible frequencies.

Tests have shown that oscillations of the threads of the gauzerelatively to each other are best suppressed by providing the threads ofthe gauze or the entire coupling member with a coating consistingessentially of a fine granular material, with very fine grains, such asstarch, as the friction of the fine grains prevents such oscillationspractically completely.

A suitable number of these coupling bodies 2%? are spaced along the gap16, their number being dependent on the length of the circumference ofthe gap and on the size which is thought to be best for the couplingmembers 213.

The coupling members 29 may be provided with axial slots or cuts oflimited length, as indicated in Fig. 4, on the free end proicctinginwardly to reduce rigidity at this end and the teeth or strips betweenthe cuts may be bent outwardly or inwardly or in different directions tominimize directional effects.

The coupling members 29 are placed astride the gap 16 in such a mannerthat their axes are preferably intersecting the small gap. The tubularmembers are then approximately at right angles to the surface of themembrane and approximately one half of the coupling member 29 extendsoutwardly and one half inwardly from said gap.

The coupling members 2% are fastened to the two sections 15, 18 of themembrane and hold these sections together. Preferably the couplingmembers are fastened to the sections 15, 18 directly without anyintermediate member and even without bending or turning over of l theedges which abut the surface of sections. The attachment is produced bymeans of an elastic adhesive. Wax or a rubber solution have both beenfound to be most suitable, but other elastic adhesives will perform aswell.

The tubular coupling bodies Zil, when attached to the inside or frontside of the membrane, as illustrated, in addition to their otherfunctions, also act as a distributing and directing means for the soundand more especially for the high frequencies.

The two sections 15, lb of the membrane may be given different inherentrigidity, but this will, as a rule not be required, as the stiffening ofthe two sections by the coupling members 2%, on account of theirdifferent positions on the sections, produces a suficient difference intheir rigidity.

As will be clear, the central or inner section 15 of the membrane isintended to radiate the higher and highest frequencies, corresponding tothe treble tones, while the outer or peripheral section 18 of themembrane radiates the lower frequencies, corresponding to the basstones. The high frequencies thus may radiate from a relatively smallsurface which may oscillate, when moved by the high frequency componentof the voice coil oscillations, without being impeded by the largesurface of the remainder of the membrane. Moreover this small surfacesection 15 is not only more rigid because it is the central section, butit is also braced to a much higher extent than the outer section by thecoupling bodies 29, which surround it and form part of its periphery.The coupling is sufficiently elastic to permit such high frequencyoscillations of the inner section without transmission to the outersection. This elimination of outer section of the membrane fromparticipation in the high frequency oscillations eliminates thedisturbances due to phase differences which always occur when theradiating surface for high frequencies is large.

The voice coil, on the other hand, can transmit all its low frequencyexcursions directly to the outer section of the membrane, as thecoupling is sufficiently stiff for such transmission.

The arrangement of a divided membrane joined by coupling membersexcludes, as already above explained, pressure reduction by partialequalization of the pressure produced at the two sides of the membraneand provides a multidirectional guide for the high frequencies which arethus better distributed through space.

Further advantages will be readily understood by those skilled in thisart. It will also be clear that unessential details may be changedwithout departing from the essence of the invention which is defined inthe annexed claims.

Having described the invention, what is claimed as new is:

l. A high fidelity sound reproducing device comprising a voice coil, acentral substantially conical membrane section closed on one side andattached to said voice coil, a separate membrane section, open on bothsides and having the shape of a truncated cone which is co-axial withand substantially forms a continu tion of the conical surface of thecentral membrane section, the central men brane section and the coaxialtruncated membrane section being separated by a gap, a plurality ofperipherally spaced coupling members arranged astride of and bridgingsaid gap, said coupling members being attached to the membrane sections,arranged substantially along a continuous conical surface, the soundoscillations of the voice coil being thus transmitted to the truncatedcone section of the membrane through the said coupling members.

2. A high fidelity sound reproducing device, comprising a substantiallyconical membrane, axially divided into an inner central section and anouter peripheral section, separated by a gap, coupling members, rigid inone direction and resilient in another direction, bridging said gap atpredetermined spaced points, said coupling members being arrangedastride of said gap and being attached both to the inner central and tothe outer peripheral section, said inner central section being attachedto a voice coil transmitting its oscillations to the outer peripheralsection through said spaced coupling members.

3. A device as claimed in claim 2, wherein the coupling members aretubular members of fibrous material.

4. A device as claimed in claim 2, wherein the coupling members aretubular members of open mesh woven fabric.

5. A device as claimed in claim 2, wherein the coupling members aretubular members of woven fabric, the fabric being covered with a coatingof fine granular material, preventing oscillation of parts of thefabric.

6. A device as claimed in claim 2, wherein the couling members aretubular substantially symmetrical bodies placed with their axesintersecting the gap, said coupling members projecting from the surfaceof the sections of the membrane substantially at right angles.

7. A device as claimed in claim 2, wherein the coupling members aretubular substantially symmetrical bodies so placed that their axesintersect the gap, said coupling members projecting from the surface ofthe sec- 8. A device as claimed in claim 2, wherein the coupling membersare tubular substantially symmetrical bodies so placed that their axesintersect the gap, said coupling members projecting from the surface ofthe sections of the membrane radially towards the inside of themembrane.

9. As an article of manufacture, a substantially conical membrane,divided along a plane at right angles to the axis of the said membrane,into two separate sections, separated by a gap, said sections being heldtogether by substantially tubular coupling members of textile fabric,approximately one half of each tubular coupling member being attached toeach section of the membrane on the inner side of the said membrane,said coupling members being placed astride the said gap and regularlyspaced along said gap.

10. A device as claimed in claim 2, wherein the coupling members aretubular members of Woven fabric, stiffened by a coating and providedwith axially directed cuts dividing the cylindrical surface near thefree end of the members, to reduce rigidity at this end.

11. A device as claimed in claim 10, wherein the strip shaped uncutportions between the cuts are bent so as to be inclined with respect tothe axis of the cylinder, in

' order to minimize directional effects.

1,833,714 Kennedy Nov. 24, 1931

